The 2023 Nobel Prize in Physics recognizes research using laser pulses to visualize the motion of ultrafast electrons. From influencing chemical reactions to advancing electronics, this second-second research has huge potential. Electrons moving through molecules may not seem like the plot of an interesting movie. But a team of scientists will win the 2023 Nobel Prize in Physics for research that essentially uses ultrafast laser pulses to track the movement of electrons, much like capturing footage with a camera.
However, electrons are part of an atom and are the glue that holds the atoms in a molecule together. They are much faster. So the tools physicists like me use to capture their motion must be very fast -- down to attoseconds.
An attosecond is one billionth of a second (10-18 seconds) - the ratio of an attosecond to a second is the ratio of a second to the age of the universe.
In photography, if you want to clearly capture a fast-moving object, you must use a fast-shutter camera or a fast strobe to illuminate the object. By taking multiple photos in rapid succession, the movement trajectory of the object can be clearly displayed.
The time scale of the shutter or strobe must match the time scale of the object's motion, otherwise, the image will be blurry. The same idea applies when researchers try to image the ultrafast motion of electrons. Capturing attosecond motion requires the use of an attosecond strobe. The winner of the 2023 Nobel Prize in Physics made groundbreaking contributions to generating this attosecond laser stroboscopic.
Imagine that the electrons in an atom are confined inside the atom by a wall. When a femtosecond (10-15 second) laser pulse from a high-power femtosecond laser is aimed at atoms of a noble gas like argon, the strong electric field in the pulse lowers the atomic walls.
This is because the intensity of the laser's electric field is comparable to that of the atomic nucleus. Electrons see this lowered wall and pass through it in a bizarre process called quantum tunneling.
As soon as the electrons leave the atom, the laser's electric field captures them, accelerates them to high energies, and knocks them back to their parent atoms. This re-collision produces attosecond laser light.
So how do physicists use these ultrashort pulses to create attosecond-scale movies of electrons?
Traditional movies are shot scene by scene, with each moment captured as a frame by a camera. These scenes are then spliced together to form a complete movie.
Electronic attosecond movies employ a similar concept. The attosecond pulses act like a strobe light, illuminating the electrons so researchers can capture their images over and over again, like a movie scene. This technique is called pump-probe spectroscopy.
However, directly imaging the movement of electrons inside atoms is currently difficult, but researchers are developing several methods using advanced microscopes to make direct imaging possible.
Typically, in pump-probe spectroscopy, a "pump" pulse sets the electrons in motion and starts filming a movie. A "probe" pulse then lights up the electrons at various times after the pump pulse arrives, so that the electrons can be captured by a "camera" such as a photoelectron spectrometer.
Information, or "images," of electronic movement are captured using sophisticated technology. For example, a photoelectron spectrometer can detect how many electrons a probe pulse removes from an atom, and a photon spectrometer can measure how much the probe pulse is absorbed by the atoms.
The different "scenes" are then spliced together to create an electronic attosecond movie. With the help of sophisticated theoretical models, these films provide fundamental insights into attosecond electron behavior. For example, researchers measured the position of charges in organic molecules at different times on the attosecond time scale. In this way, they can control the current at the molecular scale.
In most scientific research, basic understanding of a process leads to control of that process, which in turn leads to new technologies. Curiosity-driven research can lead to unimaginable future applications, and attosecond science may be no exception.
Understanding and controlling how electrons behave at the attosecond scale could allow researchers to use lasers to control chemical reactions in a way that would otherwise be impossible. This ability helps design new molecules that cannot be made with existing chemical techniques.
The ability to change the behavior of electrons enables ultrafast switching. Researchers could potentially increase the speed of electronic devices by converting electrical insulators into conductors on the attosecond scale. Currently, electronic devices process information at the picosecond level, that is, 10-12 seconds.
The wavelength of attosecond pulses is very short, usually in the extreme ultraviolet (or EUV) band, and may be used in EUV lithography in the semiconductor industry. Extreme ultraviolet lithography uses lasers with extremely short wavelengths to etch tiny circuits on electronic chips.
Recently, free electron lasers (such as the Linner Coherent Light Source at the SLAC National Accelerator Laboratory in the United States) have become bright X-ray laser sources. These lasers can now produce attosecond pulses, opening up a variety of possibilities for research using attosecond X-rays.
The idea of generating zeptosecond (10-21 seconds) laser pulses has also been proposed. Scientists can use these pulses, which are faster than attosecond pulses, to study the movement of particles such as protons within atomic nuclei.
With numerous research groups actively working on solving exciting problems in attosecond science, and the 2023 Nobel Prize in Physics recognizing its importance, attosecond science has a long and bright future.